1. Field of the Invention
The present invention is directed towards a battery charge controller for fast charging a rechargeable battery without overcharging the battery.
2. Description of the Related Art
Computers are many times needed in locations where AC power is unavailable. Rechargeable batteries are typically used as an alternative source of power, such as nickel cadmium (NiCad), and more recently nickel metal hydride (NiMH), batteries which are capable of providing power to a portable computer for several hours. Since rechargeable batteries have a limited lifespan, it is desirable to maximize the life of each battery and to obtain the maximum power from each battery during every discharge cycle. To achieve these goals, it is necessary to fully and efficiently charge the battery without causing temperature abuse and overcharging.
Many battery chargers presently in use do not efficiently charge rechargeable batteries, thereby limiting the useful life of the battery. Although some battery chargers monitor the temperature and voltage to avoid fast charging when the battery voltage is too low or the battery temperature is not within the acceptable fast charging temperature range, such as the battery chargers disclosed in U.S. Pat. application Ser. Nos. 596,223, filed Oct. 12, 1990, now U.S. Pat. No. 5,136,231, and U.S. Pat. No. 701,657, filed May 16, 1991 most known battery chargers do not have the capability to accurately determine the charge level and the total charge capacity and thus may overcharge the battery, thereby reducing its useful life. For example, many NiCad battery chargers continue fast charging until the battery voltage peaks and begins to decrease. It has been determined, however, that this negative voltage change method some times overcharges the battery, causing damage and a depression in the voltage. Typical NiMH battery chargers also tend to overcharge the battery, causing oxidizing of the hydride in the cells.
One reason for temperature abuse and overcharging of NiCad and NiMH batteries was the difficulty of determining the charge level since the voltage is substantially the same regardless of the charge level over a very broad range. Further, if the battery was removed and replaced, the system had no way of determining the charge history and the remaining charge level of the battery. Some methods estimated the charge level by monitoring the charge and discharge current through the battery over a period of time, but these methods still assumed an initial level of charge. These methods proved inaccurate when a partially discharged battery was used. Although the level of discharge may be measured for a given battery in a given system, the batteries are typically interchangeable so that if the battery was replaced, the system could not determine the charge level of the new battery. Also, rechargeable batteries tend to discharge during periods of non-use, referred to as self-discharge, so that a system which was shut down for a period of time and then powered up had no way of determining the amount of self-discharge that had occurred.
Furthermore, the total charge capacity of a battery, typically referred to as the amp-hour (AH) rating, tends to change over time and usage. For example, the charge capacity of the battery typically increases and then decreases over time and usage until the battery eventually will not hold a charge and must be replaced For example, NiCad batteries have a memory effect causing lower capacity after multiple partial charge and discharge cycles.
One particular technique used in personal computers known as Battery Watch estimates the amount of charge remaining based on elapsed time of charge and discharge, and converts this to the useful time remaining. This technique has proven unreliable since it assumes that the total charge capacity of the battery does not change over time. Further, this technique fails to keep track of the charge-in and charge-out and does not account for the self-discharge.
It is desirable therefore, to charge a battery as fast and efficiently as possible without decreasing its maximum useful life. To achieve this goal, fast charge must be terminated at the proper time without overcharging the battery. It is also desirable to accurately monitor and calculate the total charge capacity as well as the charge level at any given time, taking into account factors such as self-discharge and reduced capacity, and to provide this information to the user.